1. Field of the Invention
This invention pertains to an improved method of depositing aluminum films, and specifically, for suppressing the formation of copper precipitates in aluminum films formed from aluminum comprising small amounts of copper as a deliberately included impurity. The invention finds specific application in the field of integrated circuits and similar small dimension fabricated devices.
2. Background of the Prior Art
Metallization is a process for forming a uniform metal layer on a substrate surface, so as to take advantage of the presence of that metal layer in subsequent fabrication techniques. As applied to integrated circuit (IC) technology, metallization has come to refer to the formation of uniform layers of conductive material over a dielectric or insulating layer, or directly on a substrate. These metallizing layers provide for continuous electrical connection within an array layer, and conductivity between layers through xe2x80x9cviasxe2x80x9d, access holes penetrating through conductive and insulating layers to active regions, filled with conductive material.
The most popular metal selected for metallization in IC fabrication is aluminum. Thus, most integrated circuits begin with selection of a substrate, the formation of a dielectric, insulating layer there over, and then deposition of a metal layer, or metallization, comprised of aluminum. Aluminum is an excellent conductor, having approximately 62% of the conductivity of copper. To further improve the conductivity of aluminum and improve electromigration properties in the layer, the aluminum may be xe2x80x9cdopedxe2x80x9d or provided with a small amount of copper deliberately included as an impurity. Thereafter, a photoresist or other masking device is provided on the aluminum, and the first layer structure of the ultimate device is typically formed by etching in areas covered or uncovered by the photoresist development. Aluminum is a particularly preferred metallizing compound for a variety of reasons. Aluminum is typically deposited by physical vapor deposition (generally, through sputter deposition). Aluminum deposition at low temperatures, below about 300xc2x0 C., permits the formation of the correct aluminum microstructure, proper surface roughness, and other desirable physical-chemical properties.
In many embodiments, a wetting layer is inserted between the dielectric and the aluminum, to further improve adherence, and in particular, to ensure a uniform thickness of deposition. Exemplary wetting layers include titanium, tantalum and tantalum and titanium alloys. Tungsten has also been proposed as a metallization target and yields better uniformity of thickness when the surface being metallized has a non-flat topography. For this reason, many wetting layers are comprised of titanium/tungsten mixtures, or a TiW alloy. After the wetting layer is deposited, metallization proceeds. The wetting layer is typically much thinner than the metallization layer.
While the incorporation of copper into the aluminum metallization layer provides significant advantages in performance, low temperature aluminum deposition results in the formation of copper precipitates. These present a particular problem in IC fabrication processes, because copper precipitates frequently reach sizes of 0.1-0.5 micron, and are difficult to remove from the aluminum layer. Yet, these imperfections, if located in an area to be processed (i.e., an area to receive a feature through etching) may prevent uniform formation, interconnection, and may ultimately result in a loss of the chip. Conventional methods for removing copper pecipitates from aluminum layers are limited. The removal of copper precipitates may be effected by combining a high intensity ion bombardment with an aggressive plasma etching step. These conventional techniques, however, make it difficult to form small size features, that is, to etch and define features with very small resolution. In order to withstand these conditions, a much thicker photoresist layer is required, which itself in turn, limits the minimum resolution and size of the features to be etched. Thus, the formation of copper precipitates in copper-doped aluminum sets a process and size limitation on cold metallization formation, which is desirable from all other aspects. Those of skill in the art continue to search for a way to employ cold aluminum deposition in copper-doped aluminum, for the fabrication of IC and other small-dimensioned products, without being limited to features in excess of a certain critical size, generally about 0.3-0.4 microns.
The invention addressed herein achieves the objects described above, and other specific and valuable objects as detailed herein below. Specifically, the invention provides for a method of metallization using copper-doped aluminum, which prevents the formation of larger copper precipitates. By forcing copper back into solution, thinner photoresist films can be used, which in turn permits the formation of features of smaller dimension. Accordingly, copper precipitation is avoided as a size-limiting process feature.
The metallization process of the invention employs conventional process steps in the preparation of a substrate, dielectric layer if necessary, wetting layer if necessary and cold deposition of copper-doped aluminum. After formation of the aluminum layer, or other processing of the aluminum, the substrate bearing the aluminum layer is annealed at temperatures which force the copper in the aluminum back into solution, temperatures above about 300xc2x0 C. Specifically, in a preferred embodiment, the aluminum metallization layer is heated to its plastic deformation temperature. By holding the metallized substrate at a temperature, and for a time (typically less than a minute), sufficient to force copper precipitates formed in the deposition process back into solution, defects are avoided, yield is increased and higher resolution of small features, with relatively narrow photoresists, is achieved.